The present invention relates to a new and improved method of, and apparatus for, detecting fault currents, especially short-circuits, in electrical loads, particularly at direct-current supply lines or networks, wherein there is generated a current signal corresponding to the load current and compared with a reference value and when a difference signal formed as a result of such comparison exceeds a threshold value such is used as a triggering criterion.
The invention can be used to advantage in the transportation field, especially with direct-current or alternating-current operated railways, trolleys, trolleybuses, underground transport systems, such as subways and in other electrical networks. It is known that the indispensable protection of electrical networks against faults, predicated upon fault currents, for instance short-circuits, where the current intensity exceeds a predetermined value, can be accomplished by current-breaking means, such as high-speed circuit breakers. In this regard reference may be made to the publication "la detection des courts-circuits eloignes sur les reseaux de traction electrique a courants continus et alternatifs" by M. P. Branchu, Revue generale de 1'electricite, Volume 58, March 1949, page 103 et seq.
However, such process is not employable when the maximum permissible current intensity, during normal operation, exceeds the current intensity produced by a limited short-circuit. For instance, this is the case in an underground transport network where the current intensities prevailing during normal operation can amount to more than ten times the current intensities of the fault currents which are to be detected. If such type short-circuit is not detected, then equipment damage and accidents can arise.
Fault detection can be carried out by analysing the form of the current rise by a difference current-method, a so-called .DELTA.I protection process. In order to be able to measure .DELTA.I, the input signal must be stored or memorized before the current rise and compared with the input signal after current rise. After this comparison the storage or memory must "catch-up" with the momentary input signal. One possible solution which is used for the measurement of .DELTA.I consists in the delay of the input signal by an electromechanical element, such as a transformer having a massive magnetic circuit (damping effect) or by means of an electrical element, for instance a circuit have a T-filter. The "catch-up" of the input signal is carried out exponentially. This is disadvantages inasmuch as the .DELTA.I-value which is to be determined if falsified. In fact the "catch-up" starts from the commencement of the the rise of the signal before the latter has reached its new end value. This solution is unfavorable for the detection of remote signals, since their rise likewise is exponential and slower than that of near signals. Therefore, it is necessary to use a large time-constant in order to guarantee good operation. As a result, when signals are superimposed the system is prone to malfunction and there is a risk of unwanted triggering operations.